Method of and device for avoiding rotor instability to enhance dynamic power limit of turbines and compressors

ABSTRACT

A method of and devices for avoiding rotor instability thereby increasing the dynamic power limit of rotary fluid machines such as turbines and compressors which have contact free seals in the gaps between the rotating and nonrotating elements of the machine in which the rotary flow of fluid in the gap is modified as by axial baffles in the gap or by introducing a fluid medium into the gap thereby to decrease, eliminate, or even reverse the force component acting on the rotating element and disposed 90° ahead of the oscillation deflection of the rotating element.

This is a continuation of application Ser. No. 876,932, filed Feb. 10,1978, (abandoned), which is a continuation of Ser. No. 723,913, filedSept. 16, 1976, (abandoned), which is a continuation of 562,235, filedMar. 26, 1975, (abandoned).

The present invention relates to a method of and a device for increasingthe dynamic power limit of steam and gas turbines or compressors withseals arranged in a contact-free manner in gaps between rotating andstationary structural elements.

On rotors of thermal turbo-engines, self-excited oscillations for a longperiod have been known phenomena occurring from time to time. The outerfeature of such self-excited oscillations consists in that suchoscillations suddenly occur at a certain speed or under a certain loadof the engine which prior thereto ran quietly; also such oscillationssuddenly disappear again when the speed of the engine drops below thecritical limit. The frequency of the occurring oscillations alwayscorresponded to the first critical speed of the rotor.

The term "dynamic output limit" in the context of this application istherefore meant to be that limiting or critical output at which therotor no longer runs stably i.e. at which--from a physical point ofview--there occurs a build-up of self-excited oscillations to very largeamplitudes, due to natural excitation of oscillations of the turbine forcompressor shaft by small interferences. In other words, seen from aphysical standpoint when natural frequency of the turbine or compressorshaft occurs, in response to minor disorders, self-excited oscillationsswing themselves up to very high amplitudes.

An increase in load beyond the above-mentioned dynamic power limit isnot possible without endangering an essential part such as the bearings,the seals, and the rotor of the machine installations, it is known forpurposes of obtaining a sufficient dynamic power limit, in other words,a power higher than the maximum installed power, to dimension the rotorshafts correspondingly strong. As a result thereof, relatively heavyrotors were obtained subject to the inherent undue great use ofmaterial.

For the above mentioned self-excited oscillations of a rotor or shaftingof turbines, quite a number of possible causes have been analyzed, amongothers including the following hydrodynamic self-excitation of thejournals of shafts in the lubricating oil film of the supportingbearing; continuous excitation by elastic hysteresis and shrinkfitfriction (with rotors having wheel discs shrunk or placed thereon);subharmonic resonance due to non-linearities in spring or dampingforces; continuous excitation by bending components of the shaft torque;and continuous excitation at the steam side by the slot flows in theturbine. In this connection also a plurality of causes may occursimultaneously.

The above mentioned causes have been dealt with in literatureaccordingly merely the case of excitation by gap currents in the turbinewill be discussed in detail.

In connection with the excitation on the steam side by gap currents,heretofore only the excitation due to clearance losses was taken intoconsideration in connection with turbines. For this excitation mechanismwhich in literature is also called cap excitation, the followingconsiderations are important. With a radial dynamic deviation of therotor from the central position, as a result of the different radial gapwidth of the blades, different circumferential forces occur on thecircumference of the rotor. Such forces add to a resultantcircumferential force located on the side on which the deviation orbending occurs, and are directed perpendicularly with regard to saidbend or deviation. If the rotor is in a circumpolar condition ofoscillation, the resultant of the circumferential forces will run aheadin its direction of the bend or deflection and thus will run ahead ofthe oscillation path of the rotor precisely by 90°. The resultantlateral component of the circumferential force which in literature ismostly called transverse force is perpendicular to the momentarydirection of deviation or deflection and acts in exciting oscillation.

It is further known that the gap flows or currents in the stuffingbushings and labyrinths can generate an excitation which is similar tothe gap excitation. The influence of such similar or related excitationupon the limit of stability has, however, heretofore been consideredminor or negligible.

However, tests carried out in the meantime have proved that also fromthe course of the pressure of the flow in contact-free cells of turbinesconsiderable forces have to be expected which excite naturaloscillations. During these tests genuine labyrinth seals have been used,and the intensity of the excitation from the pressure distributionmounted at a maximum up to two times that caused by clearance losses.

The pressure distribution in eccentric seals is known and has beeninvestigated under the condition that the supply of the flow is free oftwist and no walls are in motion. With this seal passed though in apurely axial manner (disregarding symmetrical compensating currents),the pressure distribution is symmetrical with regard to the narrowestgap width in this seal or most narrow place of gap width (play). Whilein this connection a restoring force R* of minor interest occurs, notransverse or lateral force Q occurs perpendicularly or at right angleswith regard to the deviation direction and consequently no force isdependent on a deflection and excites oscillations. From the aboveresearch and tests there was derived that the cause of the actuallypresent oscillation excitation (as ascertained by the above mentionedtests) exists where the actual conditions with turbines deviate from theabove described test model. In the turbine stage, due to the test at theguiding wheel entrance as well as due to the shearing or tangentialstresses on the rotor, its discs, and shroud bands, there exists acircumferential component which acts in the direction of rotation of therotor. It is therefore assumed that the cause of the displacement of thepressure distribution from the position symmetrical to the narrowestplace, which displacement has been ascertained with turbine seals,represents such pressure distribution; in addition thereto, the mediancircumferential component of the gap current, the resultant force fromthe pressure distribution, the transverse force, and the return force asstated, are to be found in the circumferential component of the gap flowor current.

Measurements have been made that established with guide vanes, that thetransverse force caused by the pressure distribution in the flow orcurrent adds to the transverse force caused by the gap excitation sothat the normal oscillation is greatly influenced.

As heretofore the most effective step for increasing the stability limitof the rotor is known to be the increase of the critical speed bycorrespondingly stiffer and stronger design of the rotor. This step,however, has brought about in particular an increase in the diameter ofthe seals of the guiding apparatus and consequently a decrease of theinner turbine degree of efficiency.

It is, therefore, an object of the present invention to provide a methodand device for increasing the dynamic limit output.

These and other objects and advantages of the invention will appear moreclearly from the following specification, in connection with theaccompanying drawings, in which:

FIG. 1 illustrates a diagram of the pressure distribution of an"eccentric" seal which is passed through by the flow in a twist-freemanner.

FIG. 2 represents a diagram of the pressure distribution with turbineswith "eccentric" seals while a circumferential component C_(u) of thegap flow is positively defined, said circumferential component pointingin a direction of rotation ω of the rotor.

FIG. 3a represents a cutout of a central longitudinal section throughthe rotor axis with flow guiding structural elements ahead of thesealing chambers.

FIG. 3b is a section taken along the line IIIb--IIIb of FIG. 3a.

FIG. 4 is a cutout of a central longitudinal section through the rotoraxis with flow guiding structural elements within the sealing chambers.

FIG. 5 shows a cutout of a central longitudinal section through therotor axis with a thread-like entrance portion of the seal.

FIG. 6 shows a section taken along the line VI--VI of FIG. 5.

FIG. 7 is a cutout of a central longitudinal section through the rotoraxis while the gap flow circumferential component is influenced by theadmixture of flow medium.

FIG. 8 is a section taken along the line VIII--VIII of FIG. 7.

FIG. 9 is a cutout of a central longitudinal section through the rotoraxis with flow guiding structural elements ahead of the seals in the gapbetween the rotor shaft and the guiding apparatus.

The invention is based on the finding that the stability limit of anoscillatory system can also be increased by reducing the forces whichemits the oscillations and/or by increasing the oscillation dumping orreducing forces.

The problem underlying the present invention has been solved bycorrespondingly reducing--with turbines--a circumferential component ofthe gap current. This circumferential component is positively defined inthe direction of the oscillation vector of the natural oscillation, orby correspondingly increasing--with compressors--a circumferentialcomponent of the gap current. This component is positively definedcounter to the direction of rotation of the oscillation vector.

Specifically, with turbines with which the oscillation vector of theself-excited natural oscillation rotates in the same direction ofrotation as the turbine rotor, the solution according to the presentinvention consists in that a circumferential component of the gapcurrent which is positively defined in the direction of the turbinerotor is so reduced that the force component of the pressuredistribution, which component runs ahead of the oscillation deflectionby 90°, is reduced, eliminated, or reversed in its direction in thecontact-free seals.

According to a further development of the invention, flow guidingstructural elements such as deviating plates, fins, profiles, passages,or the like, are provided ahead of and/or within the gap region of thecontact-free seals, or ahead of and/or in the gap region of thecontact-free seals, a blocking or mixing medium with low, without, orwith negative circumferential component is introduced.

Due to the steps according to the invention, it is not only possible toprevent a reduction in the dynamic power limit possible in seals in viewof the pressure distribution, but it is also possible quite generally toavoid any reduction of the dynamic power output limit due toself-excited oscillations. The advantage of the step according to theinvention consists above all in that the increase in the dynamic limithas no disadvantageous influence upon the diameter of the seals of theguiding apparatus or distributor and upon the degree of efficiency ofthe inner turbine or compressor.

According to a further feature of the invention, with an anisotropicmounting of the rotor, the flow guiding structural elements or theintroduction of the blocking or mixing medium is concentrated within thelargest oscillation deflection.

Referring now to the drawings in detail, it will be seen from FIGS. 1 to8, that in the gap 1 between a fixed, i.e. non-rotating housing wall 2and the shroud band 3 of a runner 4 keyed to the shaft of a rotor of aturbine, there is arranged a contact-free seal 5 in the form of alabyrinth seal, known per se, with sealing chambers. The radiallyextending sealing plates are mortised in a two-sectional cover ring (2pointing to said cover ring). The cover ring is inserted into an annulargroove of the supporting construction for a guide wheel 10.

According to the embodiments of FIGS. 3a, 3b and 4, flow guidingstructural elements 6, 6a are arranged in the gap region 1 in front of(FIGS. 3a, 3b) or in (FIG. 4) the seals 5. The flow guiding structuralelements may be in the form of deviating plates, baffles, fins (as shownin FIGS. 3a, 3b and 4), profiles, passages, or the like. The flowconducting elements 6, 6a are uniformly distributed over thecircumference of the gaps 1. They are located in a plane extendingthrough the longitudinal axis of the rotor and are supported by thecover ring or the housing wall 2. The connection of the structuralelements 6, 6a to the cover ring or housing wall can be effected in anyconvenient manner, for instance, by welding. For the present invention,the type of connection itself is secondary.

The flow guiding structural elements 6, 6a are generally expressed, sodesigned and arranged that the mean circumferential component of the gapflow is so decreased that the force component of the pressuredistribution which runs ahead of the oscillation deflection of the rotoroscillation by 90° is reduced, eliminated, or reversed in its direction.In the last mentioned instance, for instance, simultaneously theexcitation from the clearance losses can be dumped.

The embodiment illustrated in FIGS. 5 and 6 shows a realization of theflow guiding structural elements 7a in the form of a thread-shapedentrance portion 7 of the left-hand portion of the seal. The threadextends uniformly over the circumference.

According to the embodiment illustrated in FIGS. 7 and 8, the control ofthe gap current circumferential component is effected by admixing steamor by utilizing a steam barrier with the speeds and counter reactionsnecessary therefor for impulse reasons. The steam or steam barrier isintroduced into the gap region 1 ahead of the seals 5 throughcorresponding conduits 8 uniformly distributed over the circumference.The steam or steam barrier may, however, also be introduced throughcorresponding conduits or passages in the gap region within the seals 5(not illustrated). The steps suggested according to FIGS. 3, 4; 5, 6 and7, 8 for increasing the dynamic power limit may, of course, also becombined with each other.

Similarly, the illustrated flow-guiding devices may be associated alsowith other seals than those in the gap between the shroud band and thehousing. For instance--see FIG. 9--the devices may be associated withthe seals 5' in the gap 1' between the rotor shaft 11 and the guidingapparatus or distributor 10 or (not illustrated) may be associated withthe seal of the compensating piston. The flow guiding structuralelements 6' according to FIG. 9 are arranged and designed in ananalogous manner with regard to the embodiments of FIGS. 3a, 3b and 4.Their connection is effected at the arresting guide wheel construction10.

Preferably, the flow guiding structural elements, or the feed lines forthe steam or steam barrier are associated with the seal, or the seals,which are closest to the oscillation bulge of the rotor oscillation.

With an anisotropic mounting of the rotor, the utilization of theabove-described steps (flow guiding structural elements, or the like)need not be uniformly effected on the circumference, but may beconcentrated within the region of the maximum oscillation deflection.

It is, of course, to be understood that the present invention is, by nomeans, limited to the specific showing in the drawings, but alsocomprises any modifications within the scope of the appended claims.

What is claimed is:
 1. In a method of increasing rotor stability in afluid actuated axial flow machine having radially spaced rotating andnon-rotating elements and contact-free, continuous, circumferentialradial seals in a radial gap between said elements, in which saidrotating element is subject to excessive oscillations at a critical loadso that the power output of said machine is limited to the output belowrated load, the fluid flow in said gap having a circumferentialcomponent of the gap flow to be defined positive in the sense ofrotation of a vibration vector of the natural vibrations, theimprovement comprising the step of reducing said fluid flow in thedirection of circumferential component to modify oscillations of therotating elements in said fluid flow, the vibration vector rotating inthe same sense as the direction of the rotation of the axial flowmachine wherein a second fluid flow is applied to said fluid in said gaphaving the circumferential gap flow component affected to such an extentthat the lateral force of pressure distribution in the seal issubstantially negated.
 2. The method in combination as claimed in claim1, in which the non-rotating element acts on the fluid flow through saidgap to alter the flow in said circumferential component.
 3. In a methodof increasing rotor stability in a fluid actuated axial flow machinehaving radially spaced rotating rotor and non-rotating elements andcontact-free, continuous, circumferential radial seals in a radial gapbetween said elements, in which said rotating rotor element is subjectto excessive oscillations at a critical load so that the power output ofsaid machine is limited to the output below said speed, the fluid flowin said gap having a circumferential component of the gap flow to bedefined positive in the sense of rotation of a vibration vector of thenatural vibrations, the improvement comprising the step of opposing saidcircumferential component of fluid flow to avoid self-excitedoscillations of the rotor element, a second fluid flow acts to opposethe circumferential component of said fluid flow in said gap.
 4. Arotary fluid actuated axial flow machine comprising radially spacedrotating and stationary elements having a narrow radial gap therebetweenand radial continuous, circumferential contact-free seals in said gapbetween said elements, the fluid flow through said seals having acircumferential gap flow component to be defined positive in the senseof rotation of a vibration vector of natural oscillations, said rotatingelement being subject to excessive oscillations at a critical load,thereby limiting the maximum power output of said machine, and meansacting on said fluid flow to reduce the flow of said circumferentialcomponent in the positive direction and modify oscillations of rotatingelements in said fluid flow so that the circumferential gap flowcomponent is reduced substantially, a second fluid flow acting on theflow of fluid in said gap to change the flow of said circumferentialcomponent.
 5. A rotary fluid actuated axial flow machine comprisingradially spaced rotating rotor and stationary elements having a narrowradial gap therebetween and radial, continuous, circumferentialcontact-free seals in said gap between said elements, the fluid flowthrough said seals having a circumferential gap flow component to bedefined positive in the sense of rotation of a vibration vector ofnatural oscillations, said rotating rotor element being subject toexcessive oscillations at a critical load, thereby limiting the maximumpower output of said machine, and means to oppose said circumferentialflow component of fluid, so as to avoid self-excited oscillations of therotor element so that the circumferential gap flow component is reducedsubstantially, thereby to reduce substantially to zero or to reverse thesense of direction of a resultant force produced by the gap excitation,a second fluid flow opposing said peripheral component to limitoscillations in said circumerential component.
 6. The method ofincreasing rotor stability in a fluid actuated axial flow machine havingradially spaced rotating rotor and stationary elements to leave acircumferential gap between concentric internal and external peripheriesof said elements, radial, continuous circumferential seals in the gapbetween the peripheries of said elements with continuous radial faces onsaid seals to provide a circumferential channel between saidperipheries, so that a circumferential component of flow of fluid insaid channel causes excessive oscillation of said rotor at high speed,the step of decreasing said oscillations comprising applying fluid tosaid channel in a direction opposing the said circumferential of flow.7. The method of dampening excessive oscillations of a rotary axial flowmachine in which a rotating rotor rotates within a stationary elementwith a circumferential gap between concentric external and internalperipheries of said rotor and stationary element, respectively, and acircumferential component of fluid flow confined in a circumferentialchannel between said rotating and stationary elements creates forces tocause excessive oscillations of said rotor under higher speeds, the stepof opposing the circumferential flow component in said channel to reducesaid component of flow and thereby decrease the forces acting to produceexcessive oscillations of said rotor, said step including opposing thecircumferential flow component by a second fluid flow directed againstthe said fluid flow component in said channel, thereby reducing orreversing said fluid flow component by an opposite fluid flow.